MIC4574
Micrel, Inc.
A higher feedback voltage increases the error amplifier
output voltage. A higher error amplifier voltage (comparator
inverting input) causes the comparator to detect only the
peaks of the sawtooth, reducing the duty cycle of the com-
parator output. A lower feedback voltage increases the duty
cycle.
Functional Description
The MIC4574 is a variable duty cycle switch-mode regulator
with an internal power switch. Refer to the block diagrams.
Supply Voltage
The MIC4574 operates from a +4V to +24V unregulated
input. Highest efficiency operation is from a supply voltage
below +15V.
Output Switching
When the internal switch is on, an increasing current flows
Enable/Shutdown
from the supply V through external storage inductor L1, to
IN,
output capacitor C
and the load. Energy is stored in the
The shutdown (SHDN) input is TTL compatible. Ground the
inputifunused.Alogic-lowenablestheregulator.Alogic-high
shuts down the internal regulator which reduces the current
to typically 50µA.
OUT
inductor as the current increases with time.
When the internal switch is turned off, the collapse of the
magnetic field in L1 forces current to flow through fast
recovery diode D1, charging C
.
Feedback
OUT
Output Capacitor
Fixed versions of the regulator have an internal resistive
divider from the feedback (FB) pin. Connect FB directly to the
output line.
External output capacitor C
reduces ripple.
provides stabilization and
OUT
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, connected from the
1.23V tap to FB.
Return Paths
During the on portion of the cycle, the output capacitor and
load currents return to the supply ground. During the off
portion of the cycle, current is being supplied to the output
capacitor and load by storage inductor L1, which means that
D1 is part of the high-current return path.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.23V bandgap voltage reference. The resulting error
amplifier output voltage is compared to a 200kHz sawtooth
waveform to produce a voltage controlled variable duty cycle
output.
June 2005
5
M9999-062105
MICREL [ MICREL SEMICONDUCTOR ]
